The present invention relates to a control system for a commutatorless motor, and more particularly to a control system for a commutatorless motor driven through a frequency converter in which the frequency converter is controlled to effect rapid switching between motoring operation and regenerating operation.
The commutatorless motors to which the present invention is applied are first explained in brief. They can be classified into a D.C. type and an A.C. type depending on the construction of the frequency converter. In the D.C. type, A.C. is converted to D.C. and the D.C. is converted again to A.C. to drive the motor. The D.C. type is equipped with a set of graetz-connected converters for D.C.-A.C. conversion. On the other hand, the A.C. type is equipped with a so-called cyclo-converter and it does not convert A.C. to D.C. but converts A.C. to a variable frequency and variable voltage A.C. to drive the motor. The present invention is applicable to those types of commutatorless motors.
In those motors, the motoring operation is referred to as such a driving state in which a power is fed from the A.C. power supply to the motor to drive it. In the D.C. type, the motoring is realized by operating a converter connected to the power supply (hereinafter referred to as a power supply converter) in a rectifier mode while operating a converter connected to the motor (hereinafter referred to as a motor converter) in an inverter mode. The regenerating mode is referred to as such a driving state in which the power is recovered by the power supply in order to slow down the motor speed. In the D.C. type, the regenerating is realized by operating the power supply converter in the inverter mode while operating the motor converter in the rectifier mode. By way of example, in starting the motor, the motoring operation may be continuously taken and in stopping the motor the regenerating operation may be continuously taken. When the motor speed is set to a small speed, the operation mode must be appropriately switched between the motoring operation and the regenerating operation. More particularly, in order to slow down the motor speed to a changed set speed, the operation mode is switched from the motoring operation to the regenerating operation, and when the changed set speed is reached, the operation mode is again switched to the motoring operation to maintain the changed set speed.
The switching between the motoring operation and the regenerating operation has been effected in the following manner in the conventional commutatorless motor. An example for the D.C. type is explained here.
In the D.C. type, during the motoring operation, the power supply converter is operated in the rectifier mode while the motor converter is operated in the inverter mode. When the operation mode is to be switched to the regenerating mode:
(1) The firing angle of the power supply converter is shifted into an inverter region; and
(2) After it has been confirmed that the D.C. current attenuated to substantially zero, the firing angle of the motor converter is shifted into a rectifier operation region.
When the operation mode is to be switched from the regenerating operation to the motoring operation:
(1) The firing angle of the motor converter which has been in the rectifier operation region is shifted to the inverter region; and
(2) After it has been confirmed that the D.C. current attenuated to substantially zero, the firing angle of the power supply converter which has been in the inverter region is shifted into the rectifier operation region.
In this manner, the switching between the motoring operation and the regenerating operation is effected. In any event, the phenomenon of "zero D.C. current" is used as a condition for effecting the switching. Although the explanation for the A.C. type is omitted here, it also uses the phenomenon of "substantially zero load current" as a condition for effecting the switching.
However, since the main circuit of the motor includes a large reactor such as a D.C. reactor for the D.C. type and a filtering reactor for the A.C. type, there is included a substantial amount of time delay before the load current decays. Because of the time delay due to the reactor, it has been known that the prior art system required several hundreds milliseconds for the switching.
The large time delay in switching means a slow response in changing the motor speed. That is, after a change-of-speed command has been issued, a dead time of several hundreds milliseconds is unconditionally required before the switching is finished, and it is further later when the motor speed tends to approach the commanded speed as a result of switching. Therefore, more than one second is required before the motor speed finally reaches the commanded speed. Furthermore, since a long time is needed before the effect of switching appears, a speed variance from the commanded speed is large.
Thus, because of such a slow response, the prior art system is not suitable for use with the motor used in the field in which a high precision is required. For example, in a tandem rolling mill, it has been well known that a tension is caused in a material to be rolled if there are speed deviations between the rolls. When the commutatorless motor described above is used as a roll drive motor, a tension is apt to be caused between the rolls because of large speed variance of the motor and it is difficult to recover a zero-tension condition because of slow response of the motor.
While the switching between the motoring operation and the regenerating operation for a constant speed control has been described so far, the same switching is effected in changing the established speed or reversing the motor rotation. The reversal of motor rotation is effected by switching the operation mode from the constant speed control to the regenerating operation to slow down the motor speed, and when the motor speed reaches zero, the motoring operation in reverse direction is started. In order to finish the reversal rapidly, the switching from the regenerating operation to the motoring operation must be made rapidly. Therefore, the switching delay raises a problem in this case too.